Memory chips are tested, inter alia, at the front end, i.e. at the wafer level, if the wafer has not yet been sawn into individual components and they are not yet located in housings or tested on burn-in boards. Front-end testing has the advantage that contact can be made with specific points on the chip using sharp needles and voltages or currents can be measured. Once the chip is mounted in its housing, these measuring points can no longer be accessed from the outside. Burning-in is an artificial aging of chips at a high temperature and high operating voltage. If the error rate of memory chips is plotted against their age, a characteristic curve is obtained which is similar to the shape of a bathtub, i.e. most chips are defective from the start or only have a defect after a relatively long time. For this reason, these chips are aged artificially and all components which become faulty during this process are eliminated before delivery. Burning-in is carried out here in a type of oven and, for this purpose, electrical contact must be made with the memory modules, and they are therefore located on bases which are arranged in groups on a circuit board or a burn-in board.
During normal use in a personal computer (PC), a memory chip typically has 16 or 32 pins in which data is supplied simultaneously. During the testing of these memory chips, a line must therefore be laid to each pin of the memory chip and the information flowing in it must be checked for its correctness, for example using a comparator.
During the testing of memory chips, the test mode in which the memory module knows the data (set point data) which it is intended to supply and compares it independently with stored data (actual data) has been used for some time both at the wafer level (front-end) and when testing memory chips at the burn-in-board level. If it is assumed, for example, that a logic “1” is to be stored in each cell of the memory chip but a logic “0” occurs at least once in a memory cell during the reading out of the actually stored data, then the chip is faulty. If the chip is capable of carrying out this test itself it can signal the result of the test at its pins.
As is clear from the above, the function of a plurality of memory cells is usually tested simultaneously in the memory chip, while the result of the test is usually output to fewer pins of the chip than the number of memory cells which is being tested simultaneously. In this case, the term “data compression” is used. The data compression is greater the fewer the number of pins used to output the test result. If, for example, instead of the 16 or 32 pins of the memory chip, only one pin is used for test purposes, a maximum degree of data compression is achieved. In the case of, for example, a defective memory module, the test result is then output to a single pin as a signal with a logically high state (“1”), and as a signal with a logically low state (“0”) in the case of a functionally capable memory chip. This provides a saving, for example, in terms of lines and comparators for the testing.
However, in the case of front-end testing, the result of a test is usually not transmitted to one but rather to a plurality of pins because it is thus possible to locate a fault in the memory chip more precisely. The test result for various memory areas or addresses of the chip is output at different pins. The result can generally be output at different pins depending on the address, burst bit and data output (DQ). A burst bit is a bit in which, for example in the case of a read access to the memory module, not only one information item (bit) is output per data output but rather a plurality of bits are output. This bit sequence is referred to as a “burst”. Data compression can comprise, for example, not only a plurality of data outputs but also a plurality of burst bits.
On the other hand, during the testing of memory chips on burn-in boards no value is placed on the point at which a chip is defective, but rather on whether it is defective or not so that it can then be eliminated.
Both test methods must then be integrated on one chip in order to permit a test both at the wafer level and at the burn-in-board level. It is necessary here to be able to locate the front-end test fault in the memory. This is possible only if the test result is output at a plurality of pins (relatively small data compression) of the memory module although only one chip would be sufficient for the test on burn-in boards (high data compression). However, in order to save chip area, different degrees of data compression are generally not implemented so that the test result is output at a plurality of pins, which does not constitute an optimum state for the burn-in-board level.
A disadvantage of implementing memory chips so that the test results are output at a plurality of pins of the memory chip is that, for example, a line of a comparator has to be led to each of the pins which supply test results during the test of the memory chip on the burn-in board, and the signals on the individual lines in a test device have to be compared, for example using an OR logic element if a logic “1” corresponds to a defect, in order to be able to determine the status of the memory chip.